The CDMA 1x/EVDO mobile system is a hybrid system that has evolved from the CDMA2000 system. EVDO stands for Evolution Data Only or Data Optimized and as suggested by this, is a data only system. One advantage of EVDO systems is that, it allows a high transfer rate for data. Another advantage of 1x EVDO System operating in hybrid mode is the ability to receive circuit switched services, such as voice calls, even during active data session in EVDO System.
A mobile device operating in 1x/EVDO hybrid mode will first acquire the CDMA 1x system, and once this system has been acquired and is in an idle state the mobile device will then try to acquire the EVDO system. The CDMA 1x system will always have precedence over the EVDO system according to the current design.
Since the main advantage of hybrid mode is the ability to receive circuit switched mobile terminated calls, by design, when the CDMA system is dropped for any reason, the EVDO system will also be dropped and the mobile station will immediately try to establish a new CDMA 1x connection.
On current mobile devices, a user has the option of changing the scan mode (alternatively called the network selection mode). The scan mode determines which systems the mobile device can connect to and dictates the device's System Determination Algorithm. Various options include the home network only, which prevents roaming charges, and various analog and digital options as will be known to those skilled in the art. Scan modes can be changed, for example, to avoid roaming charges when a home network can be acquired.
A side effect of changing the scan mode on current mobile devices is that they immediately drop the current CDMA network and search for a new network according to the System Determination algorithm as per the selected scan mode. In the hybrid mode that includes an EVDO connection, if the mobile device drops the CDMA 1x connection, the EVDO connection is also dropped. The problem is that when the mobile device has an EVDO traffic channel acquired and is downloading data, dropping the EVDO connection undesirably disrupts the user's download by dropping the data channel.